Question: A square and a regular heptagon  are coplanar and share a common side $\overline{AD}$, as shown. What is the degree measure of exterior angle $BAC$?  Express your answer as a common fraction.

[asy]
for(int i=0; i <=7; ++i) {
draw(dir(360*i/7+90)--dir(360*(i+1)/7+90));
}
pair A = dir(360*3/7+90);
pair F = dir(360*4/7+90);
pair C = A+dir(-90)*(F-A);
pair D = C+F-A;
pair B = dir(360*2/7+90);

draw(A--C--D--F);

label("$A$",A,NE);
label("$B$",B,W);
label("$C$",C,S);
label("$D$",F,NW);

[/asy]
Answer: The measure of each interior angle in a regular $n$-gon is $180(n-2)/n$ degrees.  Therefore, the measure of angle $\angle BAD$ is $180(7-2)/7=\frac{900}7$ degrees and the measure of angle $CAD$ is 90 degrees.  The angle, $\angle BAC$, therefore can be expressed as: \[360^\circ - \frac{900}{7}^\circ - 90^\circ = 270^\circ - \frac{900}{7}^\circ = \frac{1890 - 900}{7}^\circ = \boxed{\frac{990}{7}^\circ}.\]